This invention relates to liquid crystal and other electronic displays.
Commercially, it is highly desirable for an electronic display to be as thin and light as possible while still maintaining a high degree of ruggedness and protection against forces caused by shock, pressure, flexure, and dropping, among other things. In the area of mobile electronics, such as cell phones and personal digital assistants (PDAs), size and weight are critical factors to the commercial success of a product, but currently breakage of the displays within these devices is one of the primary reasons for repairs and product returns. In addition, the need for electronic displays that can actually be bent has been acknowledged in several areas: so-called xe2x80x98electronic paperxe2x80x99 in which fiber paper is replaced with a display would be much more compelling as a product if the electronic display could be rolled up or folded like traditional paper; wearable electronics such as computers or multifunction watches would be much more comfortable to the wearer if the display were to conform to the user""s body; chip cards which have strict flexure life-test performance standards would be able to incorporate flexible displays and still conform to those standards. Replacement of the glass substrates within displays with plastic film has been an area of active research within the display community for a number of years.
Several different technologies are under development for use in flexible plastic displays. Electrophoretic displays achieve images via electrophoreticsxe2x80x94the rapid migration of microparticles in colloidal suspensions. Light scattering particles are moved within a dyed colloidal suspension by electrostatic forces. The particles will either move toward the viewer, in which case, the typically white particles are seen by the viewer, or to the surface away from the viewer, in which case, the white particles will be hidden by the dark dye.
Cholesteric displays are another display technology being attempted on plastic substrates. When sandwiched between conducting electrodes, cholesteric liquid-crystal material can be switched between two stable statesxe2x80x94the so-called focal conic and planar statesxe2x80x94in which the liquid crystal""s helical structures have different orientations. In the focal conic state, the helical structures are unaligned and the liquid crystal is transparent. In the planar state, the helical structures"" axes are all perpendicular to the display""s surface resulting in essentially monochromatic transmission by the display.
The Gyricon display being developed by Xerox, is made of microscopic beads, randomly dispersed and held in place between two plastic sheets by a flexible elastomeric matrix of oil-filled cavities. The balls have strongly contrasting hemispheres, black on one side and white on the other. The white side is highly reflective, while the black side absorbs light. Each hemisphere has a unique intrinsic charge, resulting in a force on the ball when an electric field is applied and the axis of the ball is not aligned with the field. The side of the ball presented for display depends on the polarity of the voltage applied to the electrode. In all three of these cases, while they have some positive features such as high contrast and compatibility with plastic substrates, they all currently require high drive voltages, have slow response times, and are not compatible with commercially available drive electronics.
Liquid crystal displays (LCDs) are attractive because of the low drive voltages required to switch them, their relatively fast response times, the wide availability of drive electronics, and the significant intellectual and manufacturing investment in the technology. Attempts have been made to develop LCDs that intermixed the liquid crystal within a polymer matrix in order to make them compatible with plastic substrates, one example being polymer dispersed displays (PDLCDs). PDLCDs are fabricated by intermixing the liquid crystal and a pre-polymer into a solution prior to assembling the display. After assembling the display, the polymer is cured, typically by ultraviolet light. During the polymerization the LC separates out from the polymer into microscopic droplets. Since the droplets of LC are not in contact with any alignment layer, the orientation of the molecules is random and light is scattered by the droplets. Applying a voltage to the electrodes of the PDLCD causes the LC molecules to become aligned, resulting in the display becoming transparent. Like the other flexible displays, PDLCDs required high drive voltages not generally compatible with existing drive electronics. Prior art such as U.S. Pat. Nos. 4,688,900; 5,321,533; 5,327,271; 5,333,074; 5,434,685; 5,473,450; 5,504,600; 5,530,566; 5,583,672; and 5,949,508 all make use of phase separation of an LC/polymer mixture during polymerization of the polymer using light as the curing mechanism (photopolymerization).
Methods have been developed to achieve anisotropically dispersed LC/polymer structures which might have drive voltages lower then those achieved in PDLCDs. U.S. Pat. No. 5,949,508 describes a method in which a lamellar structure is achieved whereby the LC and polymer are disposed on opposite substrates; this reduces the drive voltages necessary to switch the device, but results in a structure where it is only practical to have the rubbed alignment surface on one of the substrates. While this structure is effective with nematic or electrically controlled birefringence (ECB) displays, it becomes more difficult to construct displays such as twisted nematic (TN) and super twisted nematic (STN) which typically require alignment surfaces on both substrates. U.S. Pat. Nos. 5,473,450 and 5,333,074 describe methods of localizing the polymer during photopolymerization by exposing only portions of the device to the light source using masks. Polymer structures of a size on the order of a pixel (xcx9c0.3 mm) are achievable, but manufacturing may be more difficult since the photomask must generally be aligned to the electrode structure within the device, and expensive collimated UV light sources must generally be employed. Structures much smaller than 0.3 mm may be difficult to achieve due to the inherent scattering of the LC/polymer mixture. U.S. Pat. No. 5,473,450 teaches the patterning of photoinitiator onto the alignment layer, but this method generally requires a highly accurate, screened deposition of the chemical photoinitiator onto the substrates. Proper alignment of the silk-screening mask to the clear ITO electrodes may be difficult to achieve, and the introduction of chemicals directly onto the polyimide alignment surface may result in poor alignment of the LC to the alignment surface, poor appearance of the display and lower manufacturing yields.
In addition to the breakage problems due to shock and drop, glass substrate displays also have difficulty surviving extremes of temperature. When the temperature of a display is cycled between cold and hot it will sometimes develop small voids between the spacers and the liquid crystal fluid. While the voids are small in size, they typically are noticeable enough that the display will be returned for repair. The voids are due to the mismatch in the thermal coefficients of expansion between the LC and the typically glass or plastic spacers. When a glass substrate display is assembled at room temperature and then sealed, its volume is essentially fixed at that point. As the display is cooled down, both the LC and spacer material will contract but due to the mismatch in the thermal coefficients of expansion and the mechanical discontinuity at the spacers, stress is localized around the spacers and voids develop. Initially, the voids are small areas of vacuum or very low pressure, but the more volatile components of the LC quickly move to a gaseous phase to fill the void to achieve a lower energy equilibrium state. When the display is returned to room temperature, the vapor filling the voids prevents the voids from being absorbed back into the LC, and the damage is typically permanent. Display manufactures have solved this problem by, amongst other methods, utilizing specially fabricated spacers that have a softer, more compliant exterior coating surrounding a core of either glass or plastic. The outer compliant layer acts to relieve the stresses encountered during thermal cycling of the display, thus preventing the voids. Because of the difficulty of manufacturing these spacers, they are often 10-20 times more expensive than regular spacers and so are often used only when absolutely necessary.
In general, the invention features a method of manufacturing a liquid crystal display device, he method comprises filling the space between two facing substrates with a mixture of pre-polymer and liquid crystal material, applying an external source of energy to initiate a polymerization, wherein as the polymerization proceeds a phase separation of the polymer and liquid crystal is generally also proceeding, after initiation of polymerization and before completion thereof, adjusting a parameter that affects one or both of the rates of polymerization and phase separation, so as to produce a variation in the polymerized structure along a direction normal to the two facing substrates, wherein the variation is a result of the rates of polymerization and/or phase separation being adjusted during the course of polymerization.
In preferred implementations, one or more of the following features may be incorporated. At least a first and second layer may be formed within the liquid crystal display device with the first and second layers overlying one another and being positioned at different locations along the direction normal to the two facing substrates, wherein the structure of the first and second layers is different as a result of the rates of polymerization and/or phase separation being different during the time that the layers are formed. The variation in the polymerized structure may change in a continuous fashion as a result of the rates of polymerization and/or phase separation being different during the time that the structure is formed. The rate of polymerization may be adjusted. The rate of polymerization may be changed abruptly so as to produce stepwise change in polymerization rate. The polymerization may be adjusted so as to produce a continuous function of time that may take the form of a ramp, an exponential curve, or other mathematical function. The intensity of a light source affecting the rate of polymerization may be adjusted to adjust the rate of polymerization. The light source may be ultraviolet light. The rate of phase separation may be adjusted. The rate of phase separation may be changed abruptly so as to produce stepwise change in phase separation rate. The phase separation may be adjusted so as to produce a continuous function of time that may take the form of a ramp, an exponential curve, or other mathematical function. The temperature may be adjusted to adjust the rate of phase separation. The first layer may be primarily polymer. The second layer may be primarily a mixture of polymer and liquid crystal material. The invention may further include a third layer, different in structure from the first and second layers as a result of the relative rate of polymerization and phase separation being different during the time that the layers are formed. The third layer may be primarily liquid crystal material. The second layer may contain droplets of liquid crystal material within surrounding polymer. The droplets of liquid crystal may be in contact with a substrate. The pre-polymer and liquid crystal may be of such a composition and the polymerization may occur under such conditions that the structure of the first and second layer can be determined by varying the miscibility of the pre-polymer and liquid crystal material. The pre-polymer and liquid crystal may be of such a composition and the polymerization may occur under such conditions that the structure of the first and second layer can be determined by varying the viscosity of the pre-polymer and liquid crystal. The viscosity may be the relative viscosity of the pre-polymer and liquid crystal. The viscosity may be the absolute viscosity of the pre-polymer and liquid crystal.